1. Field of the Invention
The present invention relates to communication systems and, more particularly, to communication systems such as two-way radios utilized by air-traffic control towers and pilots.
2. Description of the Related Art
Air-traffic control systems (that is, communications between an airport control tower and aircraft) utilize a communication system known as a half duplex communication system. In such a communication system, two-way communications between the tower and one of many aircraft in the vicinity is on the same voice frequency. In this environment, there are multiple transmitter and receivers operating on the same voice frequency such that each receiver receives every transmission. If more than one aircraft transmits at the same time or if any aircraft transmits during a control-tower transmission, then the received signal to all parties on that frequency will experience interference. In addition, the transmitting parties may not be aware of such interference and will not receive the other transmissions.
In a typical scenario, if no transmissions are occurring and if every radio is in receive mode (i.e., no one is transmitting) at the designated frequency for that flight region, then either one of the pilots or the control tower can initiate a transmission at any time. As soon as any transmission is received and heard, it is expected that no other transmission can be initiated until the airways are clear. However, a transmission may not be heard for one of many conceivable reasons, such as simultaneous transmissions, a noisy aircraft environment, a malfunctioning or intermittent headset, and so on.
The current allocation of control frequencies for air-traffic control is 118 MHz to 136 MHz, which yields a bandwidth of 18 MHz. The 18-MHz bandwidth is currently divided into 720 voice channels of 25 kHz. This current voice-channel bandwith is expected to be reduced to 8.33 kdHz (i.e., one-third of the current bandwidth). This proposed reduction in the current voice-channel bandwidth would offer the opportunity to implement many enhancements that could contribute to a more dependable and accurate communication environment.
In view of the foregoing, there is a need in the art of half-duplex communication systems or a radio that is able to indicate to a user when a transmission is taking place.